Sustainable synthesis of 5-hydroxymethylfurfural (HMF) and its direct hydrogenation to diols

Nowadays, the conversion of renewable resources to valuable chemicals and bio-fuels represents a key process in order to make the world less dependent on fossil resources. Under this perspective, 5-hydroxymethylfurfural (HMF) is one of the most important bio-based compounds that can be employed as platform-chemical for the synthesis of a wide range of fuels, monomers and other valuable products. In fact, HMF can be obtained from the dehydration of biomassderived sugars and its reactive structure makes it prone to several reactions. In this work, the synthesis of HMF and its direct hydrogenation to 2,5-bis(hydroxymethyl)furan (BHMF) and 2,5-bis (hydroxymethyl)tetrahydrofuran (BHMTHF), both promising monomers for the production of renewable polymers, were performed and optimized under sustainable reaction conditions. In particular, the synthesis of HMF was carried out starting from concentrated fructose aqueous solution in a microwave reactor employing the acid resin Amberlyst-70 as heterogeneous catalyst. The reaction was optimized and the highest ascertained HMF yield resulted 46 mol%. The produced crude HMF, for the first time, was directly hydrogenated in a batch autoclave with Ru/C as catalyst, but the presence of humins and organic acids, formic and levulinic ones, formed during the HMF synthesis were responsible for the achieving of unsatisfactory BHMF and BHMTHF yields. The introduction of a neutralizing step between the HMF synthesis and its hydrogenation was a turning point in order to increase the yields of diols. At the end, the spent catalyst was recovered and its stability was evaluated through BET and TGA analysis, which showed the decrease of catalyst surface after the reaction due to the presence of adsorbed organic matter, such as humins.